For certain applications, for example, for broad lamellae in sun shading systems, it would be desirable to make use of flat and broad, hollow profiles of extruded aluminium, but as well known it is difficult and expensive to produce such extrusions with a width exceeding some 30 cm. Of course it is possible to join such profiles edgewise, by, for example, use of joining fishes or by welding, but these methods are both expensive and unsuitable, as the ideal is to produce a reasonably smooth and uniform surface on the assembled lamella.
An immediately more attractive solution has already been proposed, namely to take advantage of the extrusion technique to the effect that the single profile plates are manufactured with tongue-and-groove-systems along the edges, such that the profiles can be joined by a lengthwise pushing together. It has been found, however, that such a joining method, which may well be realistic in connection with profile elements of short lengths, is not realistic when the length of the profiles is of the magnitude 5 m, e.g. 3-7 m. The long profile elements will have to be reasonably stiff, and even though they are produced with all care they will, however, inevitably come up with such minor wrynesses over the long length which may not be visible at all, but in connection with the stiffness of the elements make it almost impossible to use the joining by a pushing together of the elements. The engagement portions could well be shaped with suitable tolerances, but the result would be that a few elements could be joined in an easy manner for obtaining a fixed joining, while the joinings between many other elements would not be fixed, which is entirely unacceptable. In many other cases an already initiated pushing-together joining would have to be stopped, e.g. upon three fourths of the joining movement having been carried out. Possibly many of the latter joinings could be accomplished anyway by an extra strong pressing together, but this will require a quite costly pressing equipment, and there would be considerable risks of skewnesses occurring in the joined structure.
According to another known and used technique advantage is still taken of the extrusion technique, namely, by shaping the profiles with complementary, barbed clamp surface portions, which can be brought into mutual engagement by a crosswise pressing together of the elements, such that the pressing movement shall be effected over a short distance only, but it is well known that this technique presents many problems as well as a rather high percentage of waste.